Wire rod having superior hydrogen delayed fracture resistance, method for manufacturing same, high strength bolt using same and method for manufacturing bolt

ABSTRACT

The present invention relates to a wire rod used in bolts for automobile engines, for example, and more specifically to a wire rod having an improved resistance to hydrogen delayed fracture, to a manufacturing method for same, to a bolt using same and a method for manufacturing the bolt. Provided are a high strength wire rod having a superior resistance to hydrogen delayed fracture and a method for manufacturing same, a high strength bolt using the wire rod and a method for manufacturing same, wherein. 
     the wire rod comprises, 0.3-0.7 wt % C, 0.05-2.0 wt % Si, 0.7-1.5 wt % Mn, 0.01-0.1 wt % Ni, and 30-70 ppm La, and the remainder thereof is comprised by Fe and inevitable impurities.

TECHNICAL FIELD

The present disclosure relates to a wire rod used for automobile enginebolts and the like, and more particularly, to a wire rod having improvedhydrogen delayed fracture resistance, a method for manufacturing thesame, a high strength bolt using the same, and a method formanufacturing the bolt.

BACKGROUND ART

in accordance with the recent trend for weight reductions and highfunctionalization in automobiles, requirements for driving parts, inparticular, engine parts such as bolts and the like, to have highstrength, have increased in order to reduce energy consumption.Currently used high strength bolts are manufactured to have a tensilestrength of 1200 MPa through quenching and tempering processes, usingalloyed steels such as SCM435, SCM440, and the like. However, in thebolts having a tensile strength of 1200 MPa or greater, since delayedfractures may be easily caused due to hydrogen, the uses of wire rodsfor manufacturing ultrahigh strength bolts remain inadequate.

According to a bolt manufacturing process, after performing wire drawingintended for sizing, through low temperature annealing, the drawn wiremay be subjected to spheroidizing heat treatment, bolt-forming,quenching and tempering processes to finally obtain a steel having asingle-phase structure composed of tempered martensite. Thus, strengthof the bolt may be determined depending on composing, quenching,tempering and heat treatment processes performed thereon. However, thewire rod as a raw material needs to have as little strength as possiblein order to facilitate bolt-forming.

In order to highly strengthen a steel having a single-phase structurecomposed of tempered martensite, the addition of alloying elements, inparticular, carbon elements, has been known as the most effectivemethod; however, the addition of carbon may rapidly increase a ductileto brittle transition temperature (DETT) of a wire rod as well asincreasing strength of the wire rod, and remarkably deteriorate hydrogendelayed fracture resistance. In addition thereto, work hardening may beincreased, causing disadvantages in bolt-forming and a separatesoftening heat treatment may be required.

Bolts manufactured as described above may generally have a temperedmartensite structure in which carbide precipitates are distributed ingrain boundaries or gains and the basic material thereof hasprecipitates distributed in lath martensite. A main factor hindering thehigh strengthendng of the basic material may be a degradation in delayedfracture resistance due to the introduction of hydrogen, and it has beenknown because the introduced hydrogen may deteriorate the strength ofgrain boundaries. In order to use existing tempered martensite in steelfor high strength bolts, an operation for improving delayed fractureresistance may he required.

Thus, in order to achieve the high strengthening of bolts, improvementsin delayed fracture resistance may be unavoidably required to increasecritical delayed fracture strength, and to this end, a method ofgenerating precipitates capable of trapping diffusible hydrogen orcontrolling microstructure by adding certain elements while maximallysuppressing phosphorus (P) and sulfide (S) brominating austenitic grainboundaries, and the like may be present.

The related art technologies for improving hydrogen delayed fractureresistance may include 1) corrosion suppression in steel, 2)minimization of an amount of introduced hydrogen, 3) suppression ofdiffusible hydrogen contributing to delayed fracture, 4) the use ofsteel having a high concentration of limited diffusible hydrogen.contained therein, 5) minimization of tensile stress, 6) stressconcentration reduction, 7) miniaturization of austenite grain boundarysize, and the like. As a method of achieving improvements in hydrogendelayed fracture resistance, a method of implementing a high degree ofalloying, or a surface coating method or a plating method for preventingthe introduction of external hydrogen has been mainly used.

However, most inventions created domestically and internationally mayhave disadvantages such as high manufacturing costs and complexprocesses required therefor, and require excessively precise rolling andcooling conditions at the time of manufacturing steel. By way ofexample, in order to improve delayed fracture characteristics of a highstrength wire rod having a tensile strength of 1600 MPa, technologies ofadding 0.5 wt % of titanium (Ti), niobium (Nb), and vanadium (V), whichare grain refinement elements, and then, adding corrosion resistanceelements such as molybdenum (Mo), nickel (Ni), copper (Cu), cobalt (Co),and the like and carbide elements are present, but production costsrequired therefor may be significantly high, Furthermore, a method ofimproving hydrogen brittleness using ferrite structures extracted fromgrain boundaries is present, but the method does not include a chemicalcombination, and a product manufacturing cost may also increase due tothe addition of a considerable amount of molybdenum (Mo).

In addition, a technology of improving delayed fracture characteristicsof a high strength wire rod having a tensile strength of 1600 MPa orgreater, using complete pearlite is present. However, in such atechnology, 0.2 wt % or more of chrome needs to be added in order toimprove tensile strength through wire drawing and to secure drawabilityduring wire drawing intended for sizing after the production of a wirerod, and lead patenting for isothermal transformation may necessarily berequired. Thus, such a technology may have disadvantages such as highmanufacturing costs and complex processes and have limitations such asthe requirement for excessively precise rolling and cooling conditionsat the time of manufacturing steel.

Moreover, through a technology of finally securing a tensile strength of1200 to 1500 MPa using a ferrite-pearlite dual phase microstructure, thetensile strength may be secured without a final heat treatment, unlikein other technologies. However, since the technology basically aims atimproving hydrogen delayed fracture resistance by adding a greatquantity of molybdenum (Mo), it may be disadvantageous in terms of highmanufacturing costs.

As described above, limitations to a decrease in hydrogen delayedfracture resistance as compared to an improvement in tensile strength inheat-treated and non heat-treated carbon steels having a tensilestrength of 1200 MPa or greater have not yet been overcome, the securingof price competitiveness may not be available due to the addition ofexpensive alloying elements, and in particular, the stable securing ofdata regarding delayed fracture characteristics due to hydrogen may bedefective.

DISCLOSURE [Technical Problem]

An aspect of the present disclosure provides a wire rod having superiorhydrogen delayed fracture resistance while securing ultrahigh strengththrough a heat treatment, and a method for manufacturing the same.

An aspect of the present disclosure also provides a high strength bolthaving superior hydrogen delayed fracture resistance using the wire rod,and a method for manufacturing the same.

[Technical Solution]

According to an aspect of the present disclosure, there is provided awire rod having superior hydrogen delayed fracture resistance andincluding C: 0.3 to 0.7 wt %, Si: 0.05 to 2.0 wt %, Mn: 0.7 to 1.5 wt %,La: 30 to 70 ppm, Ni: 0.01 to 0.1%, and a remainder configured of Fe andinevitable impurities.

According to another aspect of the present disclosure, there is provideda method for manufacturing a wire rod having superior hydrogen delayedfracture resistance, the method including: heating steel including C:0.3 to 0.7 wt %, Si: 0.05 to 2.0 wt %, Mn: 0.7 to 1.5 wt %, La: 30 to 70ppm, Ni: 0.01 to 0.1%, and remainder configured of Fe and inevitableimpurities to a temperature of Ae3+150° C. to Ae3+250° C.; cooling theheated steel at a rate of 5 to 15° C/s and rolling the steel at atemperature of Ae3+50° C. to Ae3+150° C. to manufacture a wire rod; andcooling the rolled wire rod to 600° C. or less at a rate of 0.5 to 3°C./s.

According to another aspect of the present disclosure, there is provideda bolt including C: 0.3 to 0.7 wt %, Si: 0.05 to 2.0 wt %, 0.7 to 1.5 wt%, La: 30 to 70 ppm, Ni: 0.01 to 0.1%, and a remainder configured of Feand inevitable impurities, and having a tensile strength of 1200 MPa orgreater and superior hydrogen delayed fracture resistance.

According to another aspect of the present disclosure, there is provideda method for manufacturing a bolt having superior hydrogen delayedfracture resistance, the method including: heating steel including C:0.3 to 0.7 wt %, Si: 0.05 to 2.0 wt %, Mn: 0.7 to 1.5 wt %, La: 30 to 70ppm, Ni: 0.01 to 0.1%, and a remainder configured of Fe and inevitableimpurities to a temperature of Ae3+150° C. to Ae3+250° C.; cooling theheated steel at a rate of 5 to 15° C./s and rolling the steel at atemperature of Ae3+50° C. to Ae3+150° C. to manufacture a wire rod;cooling the rolled wire rod to 600° C. or less at a rate of 0.5 to 3°C./s; and bolt-forming using the cooled wire rod; performing a heattreatment on the formed bolt at a temperature of 850 to 950° C.; andperforming quenching after the heat treatment, and then performingtempering at a temperature of 300 to 500° C.

[Advantageous Effects]

The wire rod according to the present disclosure may be a high strengthwire rod used for the coupling of automobile components or used in suchautomobile components, and the method of manufacturing the wire rod maybe advantageous in that a wire rod having high strength of 1200 MPa to2000 MPa and superior hydrogen delayed fracture resistance, even in acase in which a tiny amount of lanthanum and nickel is added or even ina case in which a martensite microstructure is present after the finalheat treatment, may be manufactured with low manufacturing costs.

In accordance with the development of a wire rod for bolts havingsuperior hydrogen delayed fracture resistance and high strength, thestability of a steel structure may be increased due to a reinforcementof coupling force and a reduction of vacancies in a coupling part at thetime of coupling the bolts, and an amount of steel used may be reduceddue to a decrease in the number of coupled bolts. In addition, in termsof automobile components, the development of the wire rod for bolts asdescribed above may contribute to lightening of the automobilecomponents. Due to the lightening of automobile components, variousautomobile assembling device designs may be enabled and compactness ofautomobile assembling devices may be allowed.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrating a microstructure of a wire rodaccording to an exemplary embodiment of the present disclosure.

FIG. 2 is a schematic view illustrating hydrogen. trapping of amolybdenum (Mo) precipitate in the case of the addition of Mo accordingto the related art.

FIG. 3 is a schematic view illustrating hydrogen trapping of aprecipitate contained in the wire rod according to the exemplaryembodiment of the present disclosure.

FIG. 4 is a view illustrating a crystal structure of the precipitate ofFIG. 3.

BEST MODE

Hereinafter, exemplary embodiments of the present disclosure will bedescribed in detail.

First, a wire rod according to an exemplary embodiment of the presentdisclosure will be described in detail. Here, a compositional range ofthe wire rod according to the exemplary embodiment of the presentdisclosure will be described (hereinafter, referred to as weightpercentage (wt %)).

Carbon (C) may be included in the wire rod in an amount of 0.3 to 0.7 wt%, When carbon (C) is included in an amount exceeding 0.7 wt %, althoughthe wire rod may be frequently used in the form of a high carbon wirerod formed using common cold wire drawing, in a case in which the wirerod is subjected to a heat treatment suggested in the exemplaryembodiment of the present disclosure, film shaped carbides may befrequently eluted in austenite grain boundaries to thereby deterioratehydrogen delayed fracture resistance. Thus, an amount of carbon (C)exceeding 0.7 wt % may not be preferable. Meanwhile, when carbon, (C) isincluded in an amount less than 0.3 wt %, since tensile strength of abolt may be insufficiently secured through quenching and tempering heattreatments, carbon (C) may be added in an amount of 0.3 wt % or greaterin order to secure a sufficient degree of strength.

Silicon (Si) may be included in the wire rod in an amount of 0.05 to 2.0wt %. When silicon (Si) is included in an amount exceeding 2.0 wt %, awork hardening phenomenon may be rapidly generated during a cold forgingprocess for manufacturing bolts to deteriorate processability. Whensilicon (Si) is included in an amount less than 0.05 wt %, a sufficientdegree of strength may not be secured and spheroidization of cementitemay also he adversely affected.

Manganese (Mn) may be included in the wire rod in an amount of 0.7 to1.5 wt %. Manganese (Mn), an element forming a substitutional solidsolution in a base structure to perform solid solution reinforcement,may be very useful in high tension bolt characteristics. When manganese(Mn) is included in an amount exceeding 1.5 wt %, a heterogeneousstructure caused by manganese segregation may have a negative influenceon bolt characteristics, rather than having solid solution reinforcementeffects. That is, during the coagulation of steel, macroscopic andmicroscopic segregation may be easily generated according to asegregating device, and manganese (Mn) may aggravate a segregation areadue to the diffusion coefficient thereof relatively being lower thanthat of other elements and the consequent hardenability improvements maybe a main factor generating a core low temperature structure (forexample, core martensite). That is, an increase in local quenchingproperties and the formation of a segregation area caused by manganesesegregation during casting may intensify dual phase properties of thestructure.

Meanwhile, when manganese (Mn) is included in an amount less than 0.7 wt%, the segregation area may be barely affected by the manganesesegregation, but tensile strength of a final product may not be securedthrough solid solution reinforcement. That is, when manganese (Mn) isincluded in an amount less than 0.7 wt %, improvements in quenching andpermanent deformation resistance may he insufficient due to insufficientsolid solution reinforcement,

Nickel (Ni) may be included in the wire rod in an amount of 0.01 to 0.1wt %. Nickel (Ni) may be a very important element forming a compoundwithin a grain boundary, together with lanthanum (La). Thus, when nickel(Ni) is included in an amount less than 0.01 wt %, an effectivecompound, in particular, precipitates, may not be completely generated,thereby leading to an inability to improve hydrogen delayed fractureresistance, When nickel (Ni) is included in an amount exceeding 0.1 wt.%, the amount of the remaining austenite may be increased to degradeimpact toughness and manufacturing costs may be increased due to anexcessive amount of nickel.

Lanthanum (La) may be included in the wire rod in an amount of 0.003 to0.007 wt % (30-70 ppm), Lanthanum (La) may be a very important elementforming a compound within a grain boundary, together with Nickel (Ni)and decreasing phosphorous and sulfur segregated in the grain boundary.Thus, when lanthanum (La) is included in an amount less than 30 ppm, thecompound may not he effectively formed and the removal of phosphorus andsulfur segregated in the grain boundary may not be facilitated. Thus,the securing of tensile strength may be enabled but superior hydrogendelayed fracture resistance may not be expected. On the other hand, whenlanthanum (La) is included in an amount exceeding 70 ppm, sincemanufacturing costs may be increased and the hydrogen delayed fractureresistance may not be improved due to an excessive amount of lanthanum,the upper limit of the amount of added lanthanum may be 70 ppm,

The remainder may include iron (Fe) and inevitable impurities. Inaddition to the composition described above, the addition of effectiveelements may, not be excluded.

The wire rod according to the exemplary embodiment of the presentinvention may include a lanthanum (La)-based, a nickel (Ni)-based, or aLaNi-based precipitate. Types of the precipitate are not particularlylimited, but examples thereof may include LaNi₅, LaPO₄ La₂O₂S and thelike. The precipitate may he formed in a grain or a grain boundary of amicrostructure and trap hydrogen introduced into the grain or the grainboundary to prevent the introduced hydrogen from deteriorating strengthof the grain boundary, thereby improving hydrogen delayed fractureresistance.

FIG. 1 schematically illustrates a state in which precipitates aredistributed by observing the microstructure of the wire rod according tothe exemplary embodiment of the present disclosure. As illustrated inFIG. 1, it may be confirmed that precipitates of LaNi₅, LaPO₄, andLa²O₂S, are distributed in a grain or a grain boundary of themicrostructure, and a compound of LaNi₅H₆ is present due to the trappingof hydrogen.

Meanwhile, hydrogen trapping effects due to the precipitates accordingto the exemplary embodiment of the present disclosure may besignificantly superior, as compared to the related art technologyintended to improve hydrogen delayed fracture resistance throughmolybdenum (Mc). FIG. 2 schematically illustrates hydrogen trappingeffects using a molybdenum (Mo) precipitate according to the relatedart, and the molybdenum (Mo) precipitate may he intended to trapintroduced hydrogen within an interface between the precipitate and agrain to thereby improve hydrogen delayed fracture resistance. However,in FIG. 3, schematically illustrating hydrogen trapping effects due tothe precipitate according to the exemplary embodiment of the presentdisclosure, the precipitate according to the exemplary embodiment of thepresent disclosure may allow for the formation of a compound (forexample, LaNi5H₆) including introduced hydrogen, rather than confiningthe hydrogen to a surface of the precipitate, such that hydrogen presentin steel may be completely confined to thereby improve hydrogen delayedfracture resistance. Thus, in the case of FIG. 2, a defect in whichhydrogen is separated from the surface of the precipitate may bepresent, but such a defect may be fundamentally extinct, such thatsuperior hydrogen delayed fracture resistance may be obtained, in theembodiment of the present disclosure. FIG. 4 illustrates a crystalstructure of LaNi₅H₆ of FIG. 3, and it can be confirmed that thecompound of LaNi₅H₆ may have a structure capable of storing aconsiderable amount of hydrogen therein.

An aspect ratio of the precipitate may be 1.2 to 2.0. When the aspectratio of the precipitate is less than 1.2, the securing of the compoundmay rarely be allowed due to the crystal structure. When the aspectratio of the precipitate exceeds 2.0, the precipitate may be easilybroken. In a case in which the precipitate is broken in a material,continuity thereof with a base may be deficient and micro-voids may begenerated, thereby causing defects, Thus, breakage of the wire rod maybe caused and expected hydrogen delayed fracture resistance may not besecured.

Meanwhile, in terms of a size of the precipitate, a circular-equivalentdiameter of the precipitate may be 100 to 400 nm. When the diameter isless than 100 nm, the size of the precipitate may be excessively small,an amount of hydrogen trapped in the precipitate may be reduced, wherebyeffective hydrogen trapping effects may not be secured, When thediameter exceeds 400 nm, and is significantly large, since the number ofprecipitates distributed per unit area may be reduced, a decrease in asurface area of the precipitates in the overall steel may result,thereby reducing hydrogen trapping effects, the upper limit of thediameter of the precipitate may be 400 nm.

Hereinafter, a method for manufacturing a wire rod according to anexemplary embodiment of the present disclosure will be described indetail.

In order to manufacture the wire rod according to the exemplaryembodiment of the present disclosure, steel satisfying the compositiondescribed above may he heated to a temperature of Ae3+150° C. toAe3+250° C. The heating to the temperature may be intended to maintainan austenite single phase, and in a range of the temperature, austenitegrain coarsening may not be generated and the remaining segregation,carbides and inclusions may be effectively dissolved. When thetemperature exceeds Ae3+250° C., an austenite grain may be significantlycoarse, such that a final microstructure formed after cooling may behighly coarse, resulting in an inability to secure a high strength wirerod having a high degree of toughness. Meanwhile, when the heatingtemperature is less than Ae3+150° C., heating effects may not beobtained and consequently, the heating temperature may be Ae3+150° C. toAe3+250° C.

The heating may be undertaken for 30 minutes to one and a half hours.When the heating is performed for less than 30 minutes, the entiretemperature may not be uniform. When the heating is performed for morethan one and a half hours, possibility that the austenite grain may becoarse may be higher and productivity may be significantly reduced.

The heated steel may be cooled and be subjected to hot rolling. Thecooling may be performed at a cooling rate of 5 to 15° C./s and therolling may be performed at a temperature of Ae3+50° C. to Ae3+150° C.,to thereby manufacture a wire rod.

The cooling may be intended to perform controlling aiming at minimizingthe transformation of the microstructure. When the cooling rate is lessthan 5° C./s before the rolling, productivity may be decreased, anadditional device may be required in order to maintain a slow coolingrate, and further, strength and toughness of the wire rod may bedeteriorated after the hot rolling, similarly to the case in which theheating is maintained for long hours, On the other hand, when thecooling rate exceeds 15° C./s, since driving force of the transformationin steel before the rolling may be increased, the possibility that a newmicrostructure may emerge during the rolling may be increased, such thata lower rolling temperature may need to be reset.

In addition, the rolling temperature may be a temperature at which theemergence of a microstructure caused by the transformation during therolling may be inhibited, recrystallization may not be generated, andonly sizing rolling may be enabled. When the rolling temperature is lessthan Ae3+50° C., it may be close to the dynamic recrystallizationtemperature, such that the securing of the microstructure may beunavailable and general soft ferrite may be highly secured. Meanwhile,when the rolling temperature is greater than Ae3+150° C., sincereheating may be required after the cooling, the upper limit of therolling temperature may be set as described above.

The wire rod manufactured through the rolling as described above may becooled to 600° C. or less at a cooling rate of 0.5 to 3° C./s. Thecooling rate may refer to a cooling rate at which the diffusion ofcarbon may be suppressed by the addition of manganese, and the wire rodmay be effectively generated while pearlite is incompletely generatedand a sufficient area fraction is secured. When the cooling rate is lessthan 0.5° C./s, the cooling rate may be extremely low, thereby degradingproductivity to a degree to which actual work becomes infeasible. Whenthe cooling rate exceeds 3° C./s, hardenability may be improved due tooverlapping effects of the added elements, such that ferrite-pearlitetransformation may be delayed and a low temperature structure such asmartensite or bainite may be generated,

Hereinafter, a bolt according to an exemplary embodiment of the presentdisclosure and a method for manufacturing the same will be described indetail.

The bolt manufactured using the wire rod according to the embodiment ofthe present disclosure may have ultrahigh strength and at the same time,may have superior hydrogen delayed fracture resistance due to theprecipitate. The bolt according to the exemplary embodiment of thepresent disclosure may have ultrahigh strength of 1200 MPa or greaterand at the same time, may have superior hydrogen delayed fractureresistance.

The manufacturing method of a wire rod having tensile strength of 1200MPa or greater may be performed according to the following operations.First, bolt-forming may be performed using the wire rod according to theembodiment of the present disclosure, and a heat treatment may beperformed on the formed bolt at a temperature of 850 to 950° C. The heattreatment may be intended to achieve homogenization of the structurethrough austenizing. When the temperature is less than 850° C., asufficient amount of homogenization may not be performed, while when thetemperature is greater than 950° C., no further effects derived from anincrease in temperature may be secured and ductility may be deteriorateddue to the coarsening of grains. Thus, the upper limit of thetemperature may be 950° C.

After tine heat treatment, quenching may be performed and tempering maybe undertaken at a temperature of 300 to 500° C. The structurehomogenized through rapid cooling may form a low temperaturetransformation structure such as a martensite structure to therebyimprove strength of the bolt.

The tempering may be intended to control strength and improvebrittleness by removing residual stress generated due to the rapidcooling. When the temperature is less than 300° C., sufficient removalof residual stress may be difficult and rather, brittleness may begenerated as a temper brittleness phenomenon. Thus, the temperature maybe 300° C. or greater. When the temperature exceeds 500° C., thestrength may be reduced due to an excessive heat treatment, therebyleading to an inability to secure a required level of strength. Thus,the tempering may be undertaken at a temperature of 300 to 500° C.

The method for manufacturing the bolt may be intended to secure arequired level of strength by applying a common heat treatment thereto.The common heat treatment may be applied by controlling time andtemperature in order to secure strength required by a person havingordinary skill in the art and the present disclosure is not particularlylimited thereto.

MODE FOR DISCLOSURE

Hereinafter, examples of the present disclosure will be described indetail. The following examples are merely provided for understanding ofthe present disclosure, and the present disclosure is not limitedthereto.

EXAMPLE 1

Steels having compositions of Table 1 and Ae3 temperature weremanufactured and then, wire rods were manufactured using the steelsunder conditions of Table 2. Bolts were manufactured using the wire rodsmanufactured as above. In this case, heat treatment conditions in amanufacturing process of the bolts were described in Table 2.

Tensile strength and hydrogen delayed fracture resistance of therespective bolts manufactured above were measured and the resultsthereof are shown in Table 3. The hydrogen delayed fracture resistanceof the respective bolts were measured in such a manner that tensilestrengths corresponding to about 0.9 times those of tensile strength ofthe respective bolts, measured in a state in which the respective boltswere immersed in a test solution having an acidity of about 2 andconfigured of H₂O: 2000 cc, CH3COOH: 80 ml, and NaCl: 100 g wereimparted to the bolts and then, hours after which the respectivespecimens were broken, were measured. Through the test, in a case inwhich the specimen was maintained unbroken for 100 hours or more, it wasestimated that resistance to hydrogen delayed fracture was excellent.

TABLE 1 Ae3 Re- temper- Classification C Si Mn La Ni mainder atureComparative 0.01 0.02 0.55 0.001 0.06 — 886 Example 1 Comparative 0.820.5 1.2 0.05 0.06 — 856 Example 2 Comparative 0.37 0.02 0.50 — — Mo 810Example 3 0.27 Comparative 0.35 0.03 0.52 — — Mo 826 Example 4 0.64Comparative 0.40 0.02 0.55 — — Mo 823 Example 5 0.85 Comparative 0.390.18 0.78 — 0.09 — 820 Example 6 Comparative 0.44 0.55 1.16 0.004 — —825 Example 7 Comparative 0.45 0.42 1.18 0.001 0.06 — 824 Example 8Comparative 0.38 0.02 0.56 0.01 0.06 — 811 Example 9 Comparative 0.460.51 1.2 0.007 0.005 — 821 Example 10 Comparative 0.45 0.52 1.2 0.0070.15 — 824 Example 11 Inventive 0.38 0.05 0.7 0.005 0.06 — 820 Example 1Inventive 0.45 0.5 1.2 0.007 0.04 — 821 Example 2 Inventive 0.62 0.50.83 0.0048 0.06 — 785 Example 3

TABLE 2 Wire rod manufacturing process Steel heating Cooling Boltprocessing temperature Rate conditions and Cooling Rolling after HeatingTempering hour (° C., Rate (° C./ Temperature rolling Temperaturetemperature Classification Minutes) s) (° C.) (° C./s) (° C.) (° C.)Comparative 1082, 80 9.7 989 1.3 870 350 Example 1 Comparative 1090, 6213.2 956 0.2 870 350 Example 2 Comparative 1067, 72 11.8 969 2.1 870 350Example 3 Comparative 1081, 81 12.6 975 2.2 870 350 Example 4Comparative 1078, 69 13.3 958 1.9 870 450 Example 5 Comparative 1015, 7111.9 978 0.5 870 350 Example 6 Comparative 1065, 65 10.2 988 0.9 870 450Example 7 Comparative 1111, 88 9.6 990 1.5 870 450 Example 8 Comparative1093, 78 13.9 991 2.3 870 350 Example 9 Comparative 1038, 79 10.2 9720.8 870 450 Example 10 Comparative 1082, 82 11.7 965 0.3 870 450 Example11 Inventive 1053, 82 12.4 978 0.6 870 450 Example 1 Inventive 1065, 8910.2 981 1.1 870 450 Example 2 Inventive 1071, 79 9.1 980 1.7 870 450Example 3

TABLE 3 Tensile strength Breaking time (H) Classification (MPa) 10 20 3040 50 60 70 80 90 100 200 300 Comparative 1012 x x x x x x x x x x ∘ —Example 1 Comparative 1760 ∘ — — — — — — — — — — — Example 2 Comparative1390 x x x ∘ — — — — — — — — Example 3 Comparative 1420 x x x x x ∘ — —— — — — Example 4 Comparative 1435 x x x x x x x x ∘ — — — Example 5Comparative 1320 x x x x x x ∘ — — — — — Example 6 Comparative 1290 x xx x x x x ∘ — — — — Example 7 Comparative 1360 x x x x x x x x ∘ — — —Example 8 Comparative 1590 x x x x x x x x x x x x Example 9 Comparative1365 x x x x x ∘ — — — — — — Example 10 Comparative 1610 x x x x x x x xx x x x Example 11 Inventive 1250 x x x x x x x x x x x x Example 1Inventive 1680 x x x x x x x x x x x x Example 2 Inventive 2019 x x x xx x x x x x x x Example 3 ∘: Breakage Occurrence, x: BreakageNon-Occurrence

In a case in which conditions of the present disclosure were satisfied,at the time of manufacturing the bolt, it could be confirmed that thebolt had high strength of 1200 MPa or greater, while having superiorhydrogen delayed fracture resistance, However, comparative examples 9and 10 were classified as comparative examples because they hadsufficient strength and hydrogen delayed fracture resistance, but werenot preferable in terms of economical feasibility due to the addition ofan excessive amount of La and Ni.

Meanwhile, in a case in which carbon (C) was included in an excessivelylow amount as in comparative example 1, it could be confirmed that asufficient amount of strength was not secured, and in a case in whichcarbon (C) was included in an excessively high amount as in comparativeexample 2, it could he confirmed that hydrogen delayed fractureresistance was significantly low. In the cases of comparative examples 3to 5 having molybdenum (Mo) added therein, it could be confirmed thatbreakage occurred before 100 hours, the securing of sufficient hydrogendelayed fracture resistance was difficult. In the case in which only oneof La and Ni was added, as in comparative examples 6 and 7, sufficienthydrogen delayed fracture resistance was not secured.

in a case in which the addition of La or Ni does not reach a set rangeof the present disclosure, as in examples 8 and 9, it could be confirmedthat sufficient hydrogen delayed fracture resistance was not secured.

EXAMPLE 2

In order to determine hydrogen delayed fracture resistance depending ona size and an aspect ratio of a lanthanum (La)-based, a nickel(Ni)-based, or a LaNi-based precipitate, the size and the aspect ratioof the precipitate were varied through a heat treatment in the cases ofinventive examples 1 to 3.

After the size and the aspect ratio of the precipitate were varied asdescribed above, hydrogen delayed fracture resistance was measured inthe same manner as that of the foregoing example 1 and the resultsthereof were shown in Table 4.

TABLE 4 Average Aspect size ratio of of Breaking Time (H) Classificationprecipitate precipitate 10 20 30 40 50 60 70 80 90 100 200 300 RemarkInventive 320 nm 1.7 x x x x x x x x x x x x Inventive Example 1material Inventive 220 nm 1.2 x x x x x x x x x x x x Inventive Example2 material Inventive 195 nm 1.9 x x x x x x x x x x x x InventiveExample 3 material Inventive 364 nm 1.05 x x x x x x x x x ∘ — —Comparative Example material 1-1 Inventive 280 nm 3.2 x x ∘ — — — — — —— — — Comparative Example material 2-1 Inventive  97 nm 1.8 x x x ∘ — —— — — — — — Comparative Example material 3-1 Inventive 532 nm 1.55 x x xx x x x x ∘ — — — Comparative Example material 3-2 ∘: BreakageOccurrence, x: Breakage Non-Occurrence

As can be seen in Table 4, it could he confirmed that when the aspectratio of the precipitate was outside the range of the presentdisclosure, hydrogen delayed fracture resistance was low.

1. A wire rod having superior hydrogen delayed fracture resistance andcomprising C: 0.3 to 0.7 wt %, Si: 0.05 to 2.0 wt %, Mn: 0.7 to 1.5 wt%, La: 30 to 70 ppm, Ni: 0.01 to 0.1 %, and a remainder configured of Feand inevitable impurities.
 2. The wire rod of claim 1, wherein the wirerod includes a lanthanum (La)-based, a nickel (Ni)-based, or aLaNi-based precipitate.
 3. The wire rod of claim 2, wherein an aspectratio of the precipitate is 1.2 to 2.0.
 4. The wire rod of claim 2,wherein an average circular-equivalent diameter of the precipitate is100 to 400 nm.
 5. The wire rod of claim 2, wherein the precipitate is atleast one of LaNi₅, LaPO₄ and La₂O₂S.
 6. A method for manufacturing awire rod having superior hydrogen delayed fracture resistance, themethod comprising: heating steel including C: 0.3 to 0.7 wt %, Si: 0.05to 2.0 wt %, Mn: 0.7 to 1.5 wt %, La: 30 to 70 ppm, Ni: 0.01 to 0.1%,and a remainder configured of Fe and inevitable impurities to atemperature of Ae3+150° C. to Ae3+250° C.; cooling the heated steel at arate of 5 to 15° C./s and rolling, the steel at a temperature of Ae3+50°C. to Ae3+150° C. to manufacture a wire rod; and cooling the rolled wirerod to 600° C. or less at a rate of 0.5 to 3° C./s.
 7. The method ofclaim 6, wherein the heating is performed for 30 minutes to one and ahalf hours,
 8. A bolt comprising C: 0.3 to 0.7 wt %, Si: 0.05 to 2.0 wt%, Mn: 0.7 to 1.5 wt %, La: 30 to 70 ppm, Ni: 0.01 to 0.1%, and aremainder configured of Fe and inevitable impurities, and having atensile strength of 1200 MPa or greater and superior hydrogen delayedfracture resistance.
 9. The bolt of claim 8, wherein a microstructure ofthe bolt includes a lanthanum (La)-based, a nickel (Ni)-based, or aLaNi-based precipitate having an aspect ratio of 1.2 to 2.0.
 10. Thebolt of claim 9, wherein the aspect ratio of the precipitate is 1.2 to2.0.
 11. The bolt of claim 9, wherein an average circular-equivalentdiameter of the precipitate is 100 to 400nm,
 12. The bolt of claim 9,wherein the precipitate is at least one of LaNi₅, LaPO₄ and La₂O₂S. 13.A method for manufacturing a bolt having superior hydrogen delayedfracture resistance, the method comprising: heating steel including C:0.3 to 0.7 wt %, Si: 0.05 to 2.0 wt %, Mn: 0.7 to 1.5 wt %, La: 30 to 70ppm, Ni: 0.01 to 0.1%, and a remainder configured of Fe and inevitableimpurities to a temperature of Ae3+150° C. to Ae3+250° C.; cooling theheated steel at a rate of 5 to 15° C./s and rolling the steel at atemperature of Ae3+50° C. to Ae3+150° C. to manufacture a wire rod;cooling the rolled wire rod to 600 ° C. or less at a rate of 0.5 to 3°C./s; and bolt-forming using the cooled wire rod; performing a heattreatment on the formed bolt at a temperature of 850 to 950° C.; andperforming quenching after the heat treatment, and then performingtempering at a temperature of 300 to 500° C.
 14. The method of claim 13,wherein the heating is performed for 30 minutes to one and a half hours.